1. Field of the Invention
In an optical disk apparatus, information is recorded along a spiral track or concentric tracks on a recording medium, and information signals are optically reproduced by irradiating a light beam such as a laser beam. A document file system has been put into market as an early model which enabled the user to record information signals on an optical disk by a laser beam. In recent years, an optical disk apparatus serving as a peripheral recording apparatus of computers which require higher reliability has also been put to practical use. In addition, an optical disk apparatus capable of erasing and rewriting recorded information signals has been put to practical use. The technique of this type of optical disk apparatus has been applied to an optical card memory apparatus or optical tape memory apparatus which uses a card-shaped recording medium or tape-shaped recording medium.
2. Description of the Related Art
In optical recording mediums used in these optical information recording/reproducing apparatuses, a record mark with a size of about 1 micron is formed on recording tracks with a track pitch of about 1.6 microns by means of a laser beam having a beam spot diameter of about 1.2 microns. Various methods of forming the record mark have been practically used. For example, optical properties of a recording film are varied by local destruction, deformation or phase-variation. Techniques of precisely tracing record tracks by means of a laser beam include a method wherein a track guide groove is provided in advance on an optical disk and a tracking error signal is detected from a diffraction beam from the track guide groove, and a method wherein signals from marks, which are formed on both sides of a track and are slightly displaced from each other, are sampled and compared, and a tracking error signal is detected.
It is desired that these optical disk apparatuses, like other recording apparatuses, have a wider range of applications, a larger capacity and a smaller size. To meet this demand, the recording density has been increased more and more. Further, in order to realize information rewrite and overwrite, various recording materials and various film structures of recording mediums have been developed.
According to prior art, e.g. "OVERWRITE CHARACTERISTICS ANALYSIS OF PHASE-TRANSITION OPTICAL RECORDING MEDIUM", Nakamura et al., Autumn, 1990, the Digest of the 51st Applied Physics Society Symposium "Crystallizing Mechanism and Erase Ratio of Phase-variation Type Optical Disc" (The Japan Society of Applied Physics, Oct. 30, 1990), pp. 42-48, there is disclosed a recording medium wherein the thickness of a second layer and the thickness of a fourth layer in a flat multilayered structure are varied to theoretically calculate the reflectance, and the thicknesses of the layers are determined so as to obtain a desired reflectance. According to this method, the thickness of a recording material layer is set at 20 nm and the thickness of the protection layer is set at 100 nm.
According to other prior art, "CHARACTERISTICS OF REWRITE TYPE PHASE-VARIATION OPTICAL DISC", Obayashi et al., Winder, 1991, the Papers of the Second Symposium of Phase-variation Recording Research "Fundamentals and Application of Phase-Variation Type Optical Disc" (the Committee of Phase-variation Recording of the Japan Society of Applied Physics, Jan. 31, 1991), pp. 20-29, there is disclosed a recording medium wherein the thickness of a recording layer and the thickness of a protection layer in a flat multilayered structure model are varied to theoretically calculate the reflectance, and the thickness of the recording layer is determined so as to obtain a desired reflectance. According to this method, the thickness of the recording layer is set at 20 nm to 45 nm.
Calculation relating to a flat multilayered structure model is relatively easy, and it is convenient and effective for general estimation. In the prior art, a medium structure is designed from such estimation, and thereafter this structure is applied to a substrate with a tracking guide groove.
Even if a medium is formed on the basis of a structure determined by a conventional method, a desired reproduction signal modulation level cannot always be obtained. The reason for this is that an interference occurs between a phase variation of reflected light due to the tracking guide groove and a phase variation of reflected light from the recording mark region, thereby to deteriorate signals. Thus, a desired S/N cannot be obtained, and improvement in recording density and reliability is prevented.
Furthermore, when a recorded signal is reproduced from a recording medium by a conventional optical recording/reproducing apparatus, noise due to a variation in wall shape of the guide groove formed in the recording medium, i.e. so-called groove noise, is mixed in the reproduced signal, and a desired S/N of the reproduced signal cannot be obtained, and improvement in recording density or reliability of the optical recording/reproducing apparatus is prevented.